Flask



Oct. 5, 1948. .1. D. SCHWARTZ FLASK Filed NOV. 19,1946

INVENTOR /:a .22 War/z- WITNE SES:

ATTORNEY Patented Oct. 5, 1.948

UNITED STATES PATENT OFFICE FLASTK Jacob D. Sohvvartz, I'ittsb'iiigh, Pa. Application Nevemter 19, i948, serial No. #113,935

2 Claims. 1

This invention relates in general to making dental castings and more particularly to the flask used therefor.

Castings in dentistry are made principally of gold alloys and stainless steel. The dentist forms a wax patter-n, places it into a flask, orring, as it is often called, and fills the-flask with investment material. When the investment has set, heat is applied to melt and volatilize the wax pattern, thereby vforming a mold cavity in the invest ment. The flask, containing the investment material and the mold cavity therein is placed in a casting machine. The purpose of the casting machine is to drive or impel the molten gold into the mold cavity. This is done usually by centrifugal force which is generated by spinning the flask about an axis. Some machines act by creating a vacuum in the investment while others use air or steam pressure to force the molten metal into the mold. Whe'nthe molten metal has set, and the casting removed, it is usually found to be undersized due to shrinkage of the metal from its molten to solid state. In the case of gold and its alloys, the shrinkage i approximately 1.65 percent, on the linear side. In dental castings,-shrinkage of that amount is extremely appreciable and results invariably in discarding the casting and making a new one. To offset shrinkage, various forms of silica are used in the investment because when heated, they expand and enlarge the mold sufliciently to offset the casting golds shrinkage.

During extensive research in the field of dental casting, I have observed that when investment material is heated, as for exam le, in soldering the elements of a dental bridge together, the investment material develops fine fissures or cracks on its surface. This generally does not happen when the investment material is confined in a casting ring or flask with or without asbestos paper lining. This lack of cracks in the investment material is evidence that the flask in general use today, with or without asbestos lining, is restraining the investment material from attaining its total expansion when heated. This restraining action distorts the mold cavity in the investment because the investment material is very easily compressed. Consequently, the resultant inlay will also be distorted and will not, because of its inaccuracies, fit the tooth cavity.

It is, therefore, an object of this invention to provide a flask, formed of two or more complemental segments, held loosely together by rivets, whereby when the investment material expands, the segments will move apart on the rivets and 2. permit free and total e pension of the investment material.

Another object is to provide an expansible flask with internal obstructions to retain the investment material therein during handling.

Another object is to provide a flask, having elastic means "which will permit the flask to expa'nd when the investment material is expanding during Si-:ttifl'g.

These and other objects will be readily apparent from the description and the drawings in which like characters represent the same things "in the various views.

Fig. 1 is a perspective View of the flask provided in this invention, showing the flash in its expanded state.

Fig. 2 is a cross-sectional view of the flask at line II- -II, looking in the direction of the arrows, and shows the flask in its contracted state, with the ii'ask sections brought together by a rpbb'er b'ahiil surroundin the flask. 4

Fig. 3 is a cross se'ctiona'i view of the flask at line III III, looking in the direction of the arrows, and showing a wax pattern invested therein.

Fig. i is part of the rotating arm of a centrifngal casting machine showing the flask in posi- 'tl'on adjacent the crucible. m

Referring to the drawings, 1 and 2 are complemental sections composed preferably of brass 'or stainless steel stampings which form the cylindrical body of "the flask. The body, however, may he square or any form and may be formed of more than two sections. 3 and 4 are rivets, having both ends peened or flattened and fit loosely in the holes 6 and l, the distance between the flattened ends being greater than the thickness of either pair of flanges, to permit the sections 1 and 2 to be parted, as shown in Figure 1. Instead of using rivets 3 and 4, cotter pins, nuts and bolts, or any other like things may be substituted. Also, instead of flanges, the ends of the sections may be .bent to form longitudinal cylinders, with sections cut out to permit interdigitation of the ends and pins fitting loosely in the cylinders to permit expansion. 5

is an ordinary rubber band whose diameter is Just slightly less than the diameter of the flask so that the tension holding the flask sections together will not be too great, thus permitting the investment material to expand on setting without undue resistance. Instead of using a rubber band, a spring of horse shoe shape, with its ends touching the sections, may be used. Or any other elastic means may be used, just so it is of a char- 3 acter that will not hamper the sections from free and easy parting.

8 is an ordinary crucible formersprue holder.

9 is a metal sprue and I is a wax pattern.

I3 and 14 are indentations of the flask walls which prevent the hardened investment material from falling out of the flask when the flask sections are parted. The indentations must be deep enough to ofiset the distance which the flask sections are capable of being parted.

II is a crucible for melting the gold just prior to its entrance into the mold and I2 is the rotating arm of the centrifugal casting machine.

In operation, a wax pattern I5 is attached tosprue pin 9 by warming the pin and contacting same with the surface of the pattern. The free end of the pin is then inserted into the hole in the sprue holder 8. Over this assembly is placed the flask with rubber band 5 on the outside there'- of holding the two sections 1 and 2 together with their respective flanges abutting each other. Investment material is then poured into the flask until filled. Gradually, the investment material will harden and during the hardening, will expand slightly, approximately .2 percent; this is called the setting expansion. The setting expansion will bring pressure to bear against the inner surfaces of the flask sections and cause them to separate and the surrounding rubber band 5 to yield accordingly. After the investment material has thoroughly hardened, rubber band 5 is removed and also the sprue holder 8 and the metal pin 9. Heat is then applied to volatilize the wax pattern whereby the mold cavity in the investment is formed. It is to be noted here that when heat is applied to volatilize the wax pattern, the investment material will expand and will further separate the flask sections I and 2. But because the sections are free to move laterally, until they meet the rivet heads, they will move with the expanding investment material as though they were an integral part thereof. In other words, since the sections 1 and 2 are not subjected to resistance in parting from each other,

within the limits of the rivet heads, they will be parted by the expanding investment material without any effort and thereby avoid pressure on the investment which would otherwise compress the investment and distort the mold.

During handling, the hardened investment material would ordinarily slide out of the flask because of the lack of restraint from the flask sections. Therefore, the flask walls are provided with indentations l3 and M which retain the investment within the flask, and are sufliciently deep 4 to act for all degrees of parting of the flask sections. The flask is then placed in the castin machine, gold is melted in crucible H, the arm 12 is rotated, the gold, by centrifugal force enters the mold cavity and an accurate inlay, free of distortion, is formed.

Having described an embodiment of my invention in one of its preferred forms, it being understood that other form of structure can be made without departing from the spirit of the invention, I claim:

1. A flask for casting metal, comprising a hollow tubular body for holding investment material therein, formed of two or more sections joined loosely together in axial planes of said tubular body by rivets, the joined ends of said sections terminating in outwardly extending flanges, said rivets extending through said flanges and having both ends flattened, the distance between the flattened ends of said rivets being greater than the thickness of either pair of flanges, whereby the sections may be separated with respect to each other to increase the diameter of the flask and thereby accommodate expansion of the contained investment material.

2. A flask for casting metal, comprising a hollow tubular body for holding investment material therein, iormed of two or more sections joined loosely together in axial planes of said tubular body, the joined ends of said sections terminating in outwardly extending flanges and joining means extending through said flanges joining said sections loosely together whereby the sections may be separated with respect to each other to increase the diameter of the flask and thereby accommodate expansion of the contained investment material.

JACOB D. SCHWARTZ.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,043,977 Smith Nov. 12, 1912 1,372,209 Terranova Mar. 22, 1921 1,784,769 Von Dieterich Dec. 9, 1930 1,970,261 Turner Aug. 14, 1934 1,976,655 Carpenter Oct. 9, 1934 2,065,977 Jefferies Dec. 29, 1936 FOREIGN PATENTS Number Country Date 600,592 Germany July 26, 1934 

